This invention relates to tools for initiating downhole functions in a cased well at a predetermined position along the well, and methods of using such tools.
In performing operations within a cased well, such as perforating the casing at a desired depth as part of a well completion, it is important to know the exact location of the tool lowered into the well to perform the specified function. In wireline or slick line operations, the depth of the tool string is commonly determined by passing the cable over a calibrated measurement wheel at the surface of the well. As the tool is deployed, the length of cable unspooled into the well is monitored as an estimate of tool depth. Depth compensation for cable stretch may be attempted by calculating a theoretical stretch ratio based upon cable length, elasticity and tool weight. Even with very elaborate compensation algorithms, however, the actual amount of cable stretch may vary over time and because of unforeseen and unmeasured interactions between the cable and tool string and the well bore (such as tool hang-ups and cable friction) and anomalies such as cable "bounce". Deviated wells, in which the tool is pulled along the interior surface of the well casing, can present particular problems with variable and inconsistent cable loading, as the tool "sticks" and jumps along the well bore. Such problems are also encountered, albeit to a lesser degree, in tubing-conveyed operations in which tubing length is measured by a wheel arranged to roll along the tubing as it is unspooled. Even very small deployment length measurement error percentages and other discrepancies can result, with either type of deployment, in absolute tool positioning errors of several feet or more in a well of over a mile in depth, for example.
To more accurately position a tool with respect to a particular geologic formation, a combination log is sometimes prepared of a cased well prior to lowering the tool. The combination log is a correlation of two simultaneously prepared logs of a given well bore. For example, a combination log may be prepared of a geophysical parameter, such as natural gamma radiation, alongside a log of casing collars (as sensed with a casing magnetic property sensor). Such a log is sometimes called a Combined Collar Log, or CCL. The combination log is prepared by shifting the depth of one log by the fixed interval between the sensors on the logging tool to correlate the logs to a common depth reference. The usefulness of such a combination log is enhanced by the irregularity of collar spacings along the well, determined by uneven casing section lengths. After the combination log is prepared, a completion tool string equipped with a collar sensor is lowered into the well. Collar "hits" are telemetried back to an operator at the well surface as the cable is retrieved and marked every three feet or so, and the tool operator attempts to match the pattern of hits with the pattern of collars in the CCL. Matching the irregular pattern to associate a given collar "hit" with a particular collar of the CCL by visually over-laying the logs, and aided by an approximate depth indication from the cable wheel, the operator determines the exact position of the tool string with respect to the CCL, and then initiates the intended function of the tool. It is not necessary that the exact depth of the tool be determined, per se, as correlation with the CCL positions the tool relative to the geologic formation as required for optimal tool function (e.g., perforation). Although this procedure provides a more accurate positioning of the tool string with respect to the formation, it requires the direct involvement of a knowledgeable operator and must allow for both data telemetry to the well surface and remote activation of the tool string.
As oil deposits become more scarce, more accurate means of positioning tools for perforating wells for optimal recovery become increasingly important.